Control system



Jan. 9,y 1951 H. D. RooP 2,537,914

` CONTROL SYSTEM Filed June l, 1946 2 Sheets-Sheet 1 fango/lab V5h Cham bei Jan. 9, 1951 H. D. RooP 2,537,914

CONTROL SYSTEM Filed June l, 1946 2 Sheets-Sheet 2 supply voltage.

Patented Jan. 9, 1951 UNITED STATES PATENT OFFICE `CONTROL SYSTEM Harold D. Roop, Los

Automatic X-Ray Angeles, Calif., assignor to Corp., Los Angeles, Calif., a

corporation of California Application June 1, 1946, Serial No. 673,732

(Cl. Z50- 95) 13 Claims. l

My invention relates to the art of regulating voltage and has special reference to the automatic regulation of the operating voltage applied to X-ray tubes. This invention also relates to the regulation of the intensity of electromagnetic radiation, including the maintenance of constant X-ray transmission through nonuniform moving materials, a control which is essential for certain types of inspection.

The use of X-rays for irradiating materials to determine the physical and chemical properties thereof' has become a very important industrial procedure which is employed for identifying defective manufactured products, for examining engineering materials, and for medical diagnosis and X-ray therapy. Metallurgical products are tested by transmitting X-rays therethrough to determine porosity, crystal growth, slag inclusions, gas pockets, or fissures.

In testing procedures which utilize X-rays for thickness gauging and certain other forms of automatic inspection and identification or segregation of defective articles, it is desirable to regulatean X-ray generator for the purpose of maintaining a constant output level of radiation intensity for the purpose of maintaining a con- Lstant -transmission through a moving article or al'series of such articles. This necessitates very `rapid automatic modulation of the intensity of the radiation emitted by the generator, since the thickness or character of the irradiated materials mayyary quite rapidly.

Practically all conventional X-ray equipment used for industrial inspection is of the half-wave or self-rectified type in which one-half of the electrical cycle is loaded and the other haii unloaded. Conventional line voltage regulators such as the Raytheon or Sola type will not operate on this kind of unsymmetrical load. The only line voltage regulator available for such X-ray equipment is a motor driven auto-transformer actuated inversely by the variations in the line voltage for the purpose of compensating for line voltage drift and supplying constant average input power to an X-ray generator.

Although the line voltage controlled motor driven auto-transformer serves a useful purpose Where a steady X-ray output is desired, it does not hold the supply voltage within the close limits required for automatic precision gauging and it cannot compensate for any factors within the generator that may cause output intensity to vary because it is actuated only by Variations in the Furthermore, when made responsive directly to line voltage variations, it is often incapable of the quick response required in many X-ray applications where predictable instantaneous values of radiation and rapid corrective action are essential.

The present invention contemplates a system whereby the voltage applied to the X-ray generator may be Varied in response to changes in the radiation produced, e. g., the direct radiation from the X-ray tube or the radiation after transmission through an object. Heretofore no means has been commercially available for automatically maintaining a uniform intensityl of X-ray transmission through an object of varying -X-ray opacity, e. a moving object oi non-uniform thickness passing through a X-ray beam for gauging or for the detection of aWs.

An important object of my invention is to provide a system whereby the radiation emitted by an A. C. operated X-ray generator is employed to maintain the intensity of the radiation emitted therefrom at a substantially constant level or to maintain substantially constant the transmitted radiation traversing an object to be tested, e. g., an object of non-uniform thickness or character.

I have discovered that the foregoing objectives of my invention may be achieved by utilizing the intensity of the beam of radiation emitted by an X-ray generator for actuating a voltage control means, such as a regulator which controls the voltage applied to the generator. This may be accomplished by interposing in the beam of X- rays, either before or after transmission through `the device. A regulating system actuated in this manner possesses a very favorable ratio of regulation due to the fact that a given proportional change in supplyvoltage tends to cause a much greater proportional change in radiation output. It also permits the accomplishment oi certain control functions most desirable in the automatic inspection and gauging arts and which cannot be achieved by other equipment.

One of the objects of my invention is to derive a pulse of regulating energy from each pulse of radiation emitted by an X-ray generator. These pulses of radiation are emitted whenever a half-cycle oi voltage of the proper polarity is applied to the terminals of an X-ray tube. Each of such pulses or radiation has an intensity which increases from zero to a maximum and then decreases to zero, suclfifvariation being in phase with the nali-cycle of applied voltage. In the case of a selfor half-wave-rectified generator, the other half of this cycle oi applied voltage produces no radiation. The aforesaid pulse of radiation produces a corresponding pulse oi ionization current in the ionization chamber (or a corresponding pulse of phototube cu'irent in the phototube-nuorescent screen combination). Hence, an impulse of regulating energy is applied to the regulating system in synchronism with each useful voltage impulse received by the X-ray tube. Such a regulator works equally well with selfrectiiication, half-wave rectification, or full-wave rectification.

Another object of the invention is to provide a novel method and apparatus for gauging or comparing articles by use of X-ray transmission therethrough. A further object is to subject a moving object to a beam of X-rays while maintaining the intensity of the transmitted radiation ,substantially constant, preferably by control of voltage applied to the X-ray generator. A further object is to provide a novel method and apparatus in which X-ray beams simultaneously traverse an object to be tested and a standard object or device, the transmitted beams being compared as a measure of the properties or dimensions of the object being tested. Another object is to provide a novel arrangement for such comparison or beam intensities, e. g., a novel arrangement of' two ionization chambers or other radiation-sensitive devices capable both of producing an indication and of effecting regulation o i` the X-ray generator.

Further objects and advantages of` inyinvention will be evident to those skilled in the art from the following description of exemplary embodiments illustrated in the drawing', in which:

Fig. 1 is a wiring diagram illustrating the general principles of the invention in an exemplary embodiment;

Fig. 2 is a similar wiring diagram illustrating the invention as employed in the gauging or comparing of two objects;

Fig. 3 is a wiring, diagram of a circuit alter-V native to that shown in Fig. 2; and

Fig. fl is a wiring diagram of an alternative circuit employing an auto-transformer for controlorj the voltage applied to the X-ray generator.

A generalV introduction to my invention may be obtainedA by reierring to Fig. l, with the under-f standing that.v those skilled in the art will recognize the control circuit as. electrical netu work which mereiy exemplifies a suitable means of utilizing the fundamental principles ofV my discovery. Various other known control circuits may be similarly actuated and thus employed for accomplishing the objectives of my invention.

I; have discovered that an excelle-nt type regulating` energy may be obtained by interposin ionization chamber ii), or ier radiation or Zag-ray sensitive device, in the path ofv a4 beam oi" ionizing radiation, typically abeam of X-rays ii from an X-ray generator indicated generally by the numeral lil, InY some instancesthe beam li can be dimensioned by an aperture plate i3 formed oi lead or other material substantially CII opaque to the radiation, although this is not ese sential.

That portion or the beam Il reaching the ionization chamber lil may have traversed an object it, such as a standard or test specimen, and been partially absorbed and scattered thereu by. Alternatively, the ionization chamber lll may be positioned directly in the beam from the generator l2, and the radiation passing therethrough may reach an object i5, typically a standard or test specimen, since this arrangement is desirable in certain instances, as discussed hereinafter. This is one of the advantages of using an ionization chamber rather than other types of X-raysensitive devices since the ionization chamber il? may be so constructed as to intercept the entire useful beam of radiation li without interposing measurable opacity, thus making the beam available for other purposes. An ionization chamber has the additional advantage that it has a response to X-radiation which is independent of the wave length thereof and independent of voltage applied to the X-ray tube or the frequency thereof. The ionization current'reduires more amplification than the regulating current obtained from some other X-iay-sensitive devices, such as certain phototube-fluorescent screen combinations, but the necessary gain maybe readily attained without exceeding the limits of stable amplification.

The` Various components of the voltage regulator exemplified in Fig. 1 are electrically connected in such relationship that the primary voltage supplied tothe X-ray generator l2 is inversely controlled by the intensity of the beam i l. This important concept represents one of the fundamental principles of my invention Aand is utilizedv in various systems exempliied in the embodi-` ments to be described.

The X-ray generator l2 comprises a primary Winding i6, a secondary winding il inductivelf,T4 coupled thereto, and an Xray tube i8 electricallyconnected in parallel with the secondary winding; il. A modulated or regulatedvoltage is appliedf` to the primary winding` I6 by connectingl it to. a. suitable voltage control device,` exemplied as a. variable impedance or reactor 2i) having a wi-nd-. ing I9 connected in seriesr With the winding i6; and having a secondary or control winding 2|.` Theseries-'connected windings I6 and i-9,areelec. trically connected across an A. C. power lineory source of variable A. Givoltage 22. As explained' hereinafter, av suitable control means is employed to decrease the,v voltage drop across the winding;

i9 of the reactor whenever the ionization our-v rent or beam intensity decreases, and vice versa. Hence, the voltage applied to the primary winding I6 increases whenever the ionization current or beam intensity decreases, and` vice versa If an ionization chamber isl used as the radia tion-sensitivedevice, it may be of conventional', construction` or, may be of the improved type. shown in my corpending. application, Serial No.. 623,334, now Patent No; 2,458,099, issued January Il, 1949.. As diagrammatically: illustrated in. the instantdrawing, the ionization chamber prof Vides two electrodes 23;; and 2li disposed in a su-itable housing capable of transmitting X-rays. The space between the electrodes 23 and24 containsv a gas capable oi being, ionized by the bears. Ii. When an intense. electrostaticfield is maintained between the.- electrodes by a source of unidirectional voltage.. suchA asY a. battery' a5, anionization, current willaow between the eectnodes, this current varying with the intensity of the incident ionizing radiation. It should be clear that electrodes substantially transverse `to the beam Il are not requisite and that other ionization chambers employing electrodes extending in the general direction of the beam can be employed.

The invention comprehends that the resulting ionizing current be used through any suitable control means to modify the voltage applied to the winding HS oi the X-ray generator. In the instant example, a suitable control means is employed to vary the voltage control device, e. g., the variable impedance or reactor 2i), and thus vary the voltage applied to the winding i5 of the X-ray generator, preferably in a manner to maintain the ionizing radiation substantially con- Stant. If the ionizing radiation is measured alter transmission through the object i4, it will maintain the transmitted radiation substantially constant irrespective of changes in line voltages, changes in operation of the X-ray tube I or changes in opacity of the object I4.

In the arrangement shown in Fig. l, the ionization current of the ionization chamber Iii is made to control the current through the secondary or control winding 2l ci the reactor 2d, although it will be clear that other voltage control devices and other control means operatively connecting same to the ionization chamber or other radiation-sensitive device can be employed. In the villustrated control circuit, the output from the ionization chamber is led to an amplier 26, the amplifier output being rectiied by a diode 2l or other rectifying means, the rectified current Vbeing then ltered to any desired extent, as by use of a condenser 28. The voltage across the condenser is applied to a variable-resistance means, such as the winding of a potentiometer 29, having a sliding contact 3G which may be adjusted to control the biasing potential applied to the grid 3l of an amplifying electron tube 52. The grid 3i is energized with such polarity that the biasing potential becomes increasingly negative as the output voltage of the amplier 26 increases. A suitable B-potential is supplied to anode 33 of the tube 32, as indicated. The cathode 3d of this tube is connected .by a conductor 35 to a suitable phase-shifting network 36, included within dotted lines 3l.

The phase-shifting network 35 is shown as comprising a variable impedance or reactor 38 with its control winding 3d receiving the output of the tube 32 by being connected between ccnductor 35 and ground. The main winding 4S of the reactor 38 is connected by conductor Iii to a variable resistor i2 to dispose this resistor in closed circuit with the winding it and with a4 center-tapped secondary Winding 3 of a transformer lll having a primary winding [i5 energized from line 22. The center-tapped second ary winding $3 supplies an energizing A. C. voltage to the network, the resistor d2 forming the resistive arm of the network and the winding forming the reactive arm. The reactance of the winding' 'til is controlled by the plate current of the tube 32 dov/ing through control winding 3S.

The output ci the phase-shifting network 36 is through conductors il and ISB, respectively connected to the center tap of secondary winding 43 and to conductor lli. 'Ihese conductors lil' and 48 are connected to the control electrodes of a thyratron 50, namely, to cathode 5| and grid 52. The control winding 2i of the reactor is connected between anode 53 and cathode 5| of the thyratron. A resistor 55 is usud ally connected across` the: ntr'l Winding 2li for a purpose to be later described.

In effect, windings I9 and 2| of the reactor' 2t bear the same relation as primary and'secondf ary windings of a, transformer so that a flow of4 alternating current through the winding I9 will induce an A. C. voltage in winding 2 i which volt-A age is impressed between the cathode 5l and the' anode 53 of the thyratron 5B. In this connec tion, the thyratron 5S functions as a grid-con-y trolled load across winding 2|, and thus the im-` pedance of Winding I9 is decreased or increased. as the conductivity of the thyratron is increased or decreasedrespectively. The conduc` tivity of thyratron 50 is controlled by the action of the phase shifting network 3l in vary-A ing the phase angle of the thyratron ring volt-- age through the variation in the reactance of thef winding de caused by any change in amplitude of.` the plate current of the tube 32 iiowing through4 the winding 39. l

Normally, the rectified output of the amplier 25 is adjusted by the sliding contacts 30 to bias: the amplifying tube 32 just short of cut-oli'. Un der this condition, the resistor 42 in the phases shifting network 3e is adjusted for operation. of the X-ray generator I2 at any desired' or' predetermined level. Then any factor in the;- power line 22, `K-ray generator I2, or object; ill tending to cause a change in this predetermined level also tends to cause a change :in the ionization current which, inturn, alters the bias.- ing potential of the grid 3| of the amplifying tube If there is a decrease in radiation output from the predetermined level, the biasing poten tial of thegrid 3i of thetube 32 becomes less: negative, which increases the plate current ofi the tube. This increased plate current nowing: through the winding 39 decreases the reactance.` of the winding 49, which has the effect of shift-I ing the phase angle of the ring voltage applied to the grid of the thyratron 5i! in such directionv as to increase its effective conductivity. They increased conductivity of the thyratron 50 re duces the reactance of the winding I9, which. causes a greater voltage to be applied to the: primary winding I5 of the X-ray generator l2'. This action, which` is extremely rapid, continues until the intensity of radiation entering ionization chamber i3 is restored to the value that existed before the change in output occurred. The opposite action takes place if the X-vray generator output rises above thev predetermined level. Hence, the effective voltage applied to the X-ray generator l2 varies inversely with the ionization current in the ionization chamber I0 in such a way as to maintain at a constant level the intensity of radiation entering the chamber.

The control circuit described hereinabove is suitable for regulating the voltage applied to half-wave or self-rectied X-ray generators sinceV but one thyratron is employed, the reactor 2B being thus saturated during half of each cycle. Incidentally, in the case of self-rectified generators, this control circuit serves as an excellent inverse voltage suppressor, through the action of the resistor connected across the winding 2l and which passes enough energy to collapse the X-v ray transformer eld during the unused portion of the cycle. This control circuit can be adapted for use with full-wave rectifying equipment by adding a second thyratron and removing the by,- pass resistor 55.

The circuit of Fig. 1 has a. desirany hier ratio carefully repeated measurements oi radiation intensity. In many instances, e. g., inthe gauging oli-*automatic inspection of materials, manual compensation is entirely inadequate. K

The system of Fig. 1 can be used for gauging by' measurement of currents or voltages'iny various portions of the circuit as an indication of the dimension or characteristicV to be determined. For example, the object i4 may be a moving strip of material or different specimens may be placed inthe beam to obtain an indication-of the thickness or X-ray opacity, the latter being indicative of the character and position oi defects.

However, a superiorl inspection, e. g., gauging orvtesting for flaws, can be achieved by employ'- ing the circuitsof Figs. 2 or 3i In addition, such systems canv be used, if" desired, to compare a test'. specimen with a standard.

Figa 2 shows, as before, the-X-ray generator It and its related control equipment, the elements being indicated by the same numerals as in= Fig. 1' sou far as applicable. In the system of Fig;` 2,' however, the totalemission of.V the X-ray tube 18 is divided into, or produces in effect, two beams' 70 and 1I, respectively directed.: toward ionization chambers 'l2I4 and "i3, or other radiation-sensitivedevices. li desired, the total emission fronilthe-tube i8 can be divided-into two separate beamsl by suitable aperture means, not shown. The ionization chamber 'E2 is shown as including parallel electrodes i4 and l5, exem- 'plied as substantially perpendicular to the incidentrradiation of thev beam lil. Similarly, the ionization chamber 13' is shown as including electrodes i6 and 1l. Articles or: objects SQ and 8| are shown as disposed respectively in the beams 'H1 and 'H'. To exemplify some of* the principlesl and advantages of the invention, the objects Sit' and 8i are shown as wedge shaped and V being synchronously movable through therespective beams in the direction ofY arrow 82, as by being pushed by an oper-ating member 8'3. The upper faces of the objects t and 8l are shown as substantially perpendicular to the incident radiation.

The electrodes of the ionization chambers 'i2 and I3 are here connected to a suitableinspection bridgecircuit, indicated generally by the numeral 35. In the simplified circuit shown, the ionization chambersl and i3 are respectively disposed in arms 8% and 810i the bridge, the

remaining arms being constituted by end sections of' a potentiometerwinding 63. The previously described source of unidirectional potential 25 is connected between variable arm of the potentioiz'ieter the upper junction. of the bridge to form one diagonal of this bridge. Across the other diagonal is connected a suitable indicating means, exemplified as a sensitive galvanometer 9G. if the object Sti is to be considered as a standard andthe object 8| as a test specimen, the junctionel ofv the. bridge` is connected to theamplier 26the remaining portionl oithecircuitbeing as before.

The operationV of. the circuit of Fig.v 2 will: be discussed under several conditions; Assume.; rst, that the control circuit is rendered: inoperative to maintain the. desired beam intensity, as by.l disconnecting the. ampliiier-v 26 andi that the objects. Siland 8l are. of' uniform thickness throughout their length. Under such conditions, the. object maybe merely an appropriate4 ra,- diation iilter matched: in X-ray: opacity` to that of the specimen St whenV of' desired thicknessorcharacter. Ii' the bridge isv preliminarily balanced, by adjustment of the. potentiometer 88 while anv object. 8i of desired opacity is.A present, subsequent insertions into ther beam 'H 0f other test objects or' specimens will. effect. gauging thereof or comparison thereof. with the standard object 8e. For example.,theebridgefwill be in balanceV when the object. 8.1. is of the: de..- sired standard thickness or itsr opacity corre"- spends tov that desired. If. either more,A or less radiation transmitted through the object 8.! (e. g., the object is: thinner or thicker than standard) the bridge will indicate; av corresponding unbalanee and the degree of the unbalarice will be shown directionally onthe. galvanometer et.. Similarly, if the object. 8l is of substantially uniform thicknessl and is moved alongthe beam' ii, its thickness or character at Various sections can be compared with the standard. If the level oi X-ray output from the tube I8 is kept constant, it. is possible. tov calibrate the galvanoineter et in units of thicknesslof the objecty Si beinggauged.

However, evenl small changes in line voltage may produce marked inaccuracies, as will beLunf derstood from the following discussion.. If. the energyA flowing in the bridge under average con.- ditionsbe arbitrarily assigned a value ofi 100, and it be assumed that a variation of 1% from standard thickness will cause an unbalance' of unity, then a change in line voltage suicient to increase the intensityy of the transmitted radiation 25% will cause a 1% variation from standard thickness to register as a 1.25% variation. Under average conditions, a change in 16% of line voltage may double or halve the transmission, while a change of 5% is often more than enough to change the transmission 25%. Under fixed conditions, an X-rayethickness gauge is a sensitive, precise instrument capable of measuring. variations from standard thickness. smaller than 0.2% or one part in 500. However, this sensitivity cannot be achieved With a generator whose output is affected by the usual variations in line voltage since the latter usually amount to several per cent even on the. best. regulated power lines.

if, under the above conditions, the bridge4 is connected to the ampliiier to render operative the control circuit, the voltage applied to the X- ray generator i?. 'will be adjustedto compensate for suchv line variations, and any variations inherentin the X-ray generator I2,.to the end that the transmission through the object 8i): will be maintained substantiallyv constant. Under. such conditions, the invention will maintain an X-ray output whose variationu is butV a fraction of. 1% even if the line voltage varies as greatas 20%..

in the industrial Xfray testing of objects, many such objects have awcdge-.like configuration from an Xnay pointofvew. Among somt,l

objects are hollow steel airplane propellers, spar caps for airplanes, ordnance shells, rockets, etc. For this reason, Fig. 2 shows objects Bil and S of wedge shape to illustrate the operation of the `invention in the inspection of objects having non- `uniform cross sections, either for gauging or `testing for flaws. In almost all cases the standards of inspection are based upon the proportion existing between the size of a `flaw and the thickness of the section at the point where the flaw occurs. These standards are usually expressed in percentages of the total thickness.

f The impossibility of achieving satisfactory results with conventional apparatus, when inspecting objects of non-uniform cross section, may be illustrated by the following arbitrary example. Assume that a simple wedgeis to be inspected, .the wedge being formed of aluminum and having uniform width and a thickness which varies from 4" at the butt to l at the tip. Assume 4that the inspection specifications state that a variation of 5% or more from normal at any point between the butt and tip represents a rejectable object, whether the defect is due to voids. inclusions, or other causes, but that lesser variations are acceptable. A similarly dimensioned aluminum wedge free from flaws will be assumed as used as a standard in the other X-ray beam. Assume, first, that the control circuit is rendered inoperative, as by disconnecting the amplifier 26.

As the standard wedge, representing the object I8f3, is moved through the beam 1l] and the wedge to be tested, representing the object 8|, is synchronously moved through the beam 1I, the bridge circuit will indicate. to some extent, flaws or abnormalities in the test wedge. However, the intensity of the incident radiation will be substantially constant, assuming no line voltage variations, and the transmitted radiation through the two wedges will vary markedly from one end of the wedge to the other. The most diflicult portion of the wedge in which to reveal a 5% defect is the thick end. Practical experience dictates the need for an X-ray tube voltage of about 200 kv. p. for this 4 thickness of aluminum. Assume that the energy flowing in the bridge is defined as having a level of 100 when the inspection starts at the butt end and that a 5% defect will cause an unbalance of lve to be indicated by the inspection bridge circuit 8e. Assume, also, that the beams reaching the two `ionization chambers `are confined, by suitable sans, to be relatively narrow in the direction of the length of the moving wedges.

2, Under such circumstances, the "synchronous translation of the Wedges through the bea-ins causes the intensity of the transmitted radiation to increase rapidly as the thickness of the wedges decreases. The amount of this increase may be determined by reference to X-ray transmission curves for aluminum sheets of various thickness, which show that the X-ray transmission will increase 550% as the thickness decreases 50%. Hence, at a pointwhere the wedge is 2 thick, the energy owing in the inspection bridge circuit will be increased to a level of 550 and, consequentlyja 5% defect at this point will cause an unbalance equivalent to that produced by a 26.5% defect in the 4" section. In other words, a defect of less than 1% will register here as a 5% defect. Finally, at a point where the wedge is 1" thick,` the energy level in the bridge will be increased to 1400 and a 5% defect will appear to be a '10% defect' or, stated` in another way, a

10 defect of 1% will register here as 14%. The overall results are thus not usable.

However, if the effective energy level in the inspection bridge circuit 85 is maintained uniform, as by connecting the amplifier 2t and the control circuit as suggested in Fig. 2, very satisfactory inspection can be obtained. This is true even though the relationship between transmission and thickness is not linear and even. though this relationship is dependent upon thenature of the material being tested or the complex shape of the object. When using the present invention, the intensity of the transmitted rediation is automatically maintained substantially constant by adjustment of the supply voltage to the X-ray `tube |18. These conclusions follow whether` the objects Bil and 3l are wedge shaped or whether they have irregular thickness or variable opacity at diiTerent sections.

The regulation system is preferably made responsive to the radiation passing through the standard object, e. g.. the object 80. The invention continuously adjusts the output of the. X- ray generator l2 to maintain the radiation intensity reaching the ionization chamber 12 substantially constant. The intensity of radiation reaching the ionization chamber '13 will vary with the transmission chars cteristics of the test obiect and the bridge will indicate accurately the differences between the two objects at related sections thereof. Consequently, by means of the invention. the inspection or gauging of complex materials, e. g., materials having variable chemical or physical` characteristics. is rendered as simple and reliable as the inspection or gauging of uniform sheets of homogenous material.

.1- greater sensitivity.

`The embodiment of Fig. 3 illustrates a different inspection bridge circuit, outined by dotted lines |59 and which has the advantage of much A circuit of this type' is claimed specifically in my copending application, Serial No. 623.335, now Patent No. 2.513.818. issued July 4, 1950, and referencethereto will indicate additional advantages and characteristics over and above those herein briey described.

Referring to Fig. 3. one terminal of the source of unidirectional potential 25 is grounded at lill. As before, this source enerffizes both ionization chambers. The output of the ionization cham-- ber 73 is delivered to one end of a potentiometer Winding i072, the potentiometer havingr a variable contact H13 connected to ground through potentiometer winding IM, this potentiometer having a variable Contact H15. The output of the ionizan tion chamber 'I2 is delivered to the input of the amplier 25 as before and also through potenu tiometer winding Hl'l to ground. this potentiometer having a variable contact IBB. This variable contact is connected to the grid of an electron tube lli), which acts as a pbase-shifting tube. The cathode of this tube is biased by a resistor lll. The tube lill provides an anode or plate H3 to which a suitable plate potential is applied through resistor IM. lill is delivered through condenser lili to the remaining end of the potentiometer Winding 1132 and appears there, by virtue of the characteristic action of the tube |l0.` out of phase withv the input supplied by the ionization chamber 13.

The variable contact Yarm l H8 is adjusted so that with equal signals supplied by the ionization chambers. the output of the tube lill. appearing at one end of the potentiometer winding H32. is equal to the signal appearingr at the other end of the potentiometer winding-1102 from the ioniza- The output of the tube` 13 type, the armature |31 being connected in series with one of the conductors 22 of the line, this conductor extending to the blade |15. The remaining line conductor 22 extends to the blade |16.

The operation of the system shown in Fig. 4 is as follows. The sliding contacts |43 and IM are adjusted on the resistor |42 until the relay |450- is open and the relay |51 is closed, as shown.

This is done during the time that the intensity of the X-ray beam is at the desired level. Adjustment of the contacts |43 and |44 changes the grid potentials of the triodes. Due to the relationship of the contacts |43 and 444, the grid |46 of the triode is always less negative than the grid |45 of the triode |56. With this setting of the relays, both the armature and neld circuits of the motor |35 are open so that the armature |31 does not turn.

If the intensity of the X-ray beam increases, the grid |46 becomes more negative thus decreasing the anode current through the winding |59 and opening the relay ll, the relay |60 remaining open. At this time current flows through the armature |31, blade |15, contacts |19 and |59, blades and |66, contact |18 and conductor |81 to the lower terminal of the field winding |36. The return circuit is from the upper terminal of this iield winding through the conductor |39, contact |8i3 and blade |16. This energizes the field winding |36 in one direction and causes the motor to turn in a forward direction to decrease the voltage supplied to the X-ray generator. This reduces the intensity of the X-ray beam to the normal level, at which time the relay closes to deenergize the motor.

Conversely if the intensity of the X-ray beam falls below normal, the potential of the grid |45 becomes more positive with the result that the anode current in the triode |53 increases to close the relay |50, the relay |5| remaining closed. Current then ilows through the armature |31, blade |15, contacts |8| and |1|, blades |55 and |55, contact |12 and conductors |911 and |39 to the upper terminal of the eld Winding 236. The return circuit is from the lower termina-i of this field winding through conductors |81 and |88, contact |82 and blade #15. This reverses the current through the iield winding, as compared with the previous example, and rotates the armature |31 in the opposite direction to increase the voltage applied to the X-ray generator. As before, the motor |35 stops when the intensity becomes normal and the relay Ii opens.

It should be understood that various radiationsensitive devices other than ionization chambers can be employed. It should also be clear that voltage control devices other than a saturable reactor or a reversible motor can be employed and that other types of control means can be employed to vary the voltage control device in response to signals from the radiation-sensitive device. The illustrated arrangements are merely 1'4 art and which are comprehended in the scope of the appended claims.

I claim as my invention:

l. In combination in a system for maintaining substantially constant the radiation intensity of an X-ray generator at a given position in the beam of X-rays produced thereby: radiationsensitive means at said given position and responsive to variations in radiation intensity of said beam at said position; and means for controlling the voltage applied to said X-ray generator to maintain said radiation intensity at said position substantially constant, said means comprising voltage control device for controlling the voltage applied to said generator and including a control means operatively connected to said radiation-sensitive means and said voltage control device for varying said voltage applied to said generator to maintain said radiation intensity at said position substantially constant.

2. A combination as defined in claim 1, in which said voltage control device comprises a variable impedance including a winding connected to said X-ray generator to vary the voltage applied thereto and a control winding, said windings being inductively coupled, and in which said control means includes means for changing" windings being inductively coupled, and in which" said control means includes a grid-controlled tube for varying the current flowing through said control winding and` means operatively connecting the grid of said tube to said radiationsensitive means.

4. A combination as dened in claim 1, in which said voltage control device comprises a variable impedance including a winding connested to said X-ray generator to vary the voltage applied thereto and a control winding, said windings being inductively coup-led, and in which said control means includes a grid-controlled tube for varying the current owing through said control Winding and a phase shifting network operatively associated with said radiation-sensitive means for controlling the grid voltage of said tube.

5. A combination as dened in claim l for the comparative inspection of a test object and a standard object and `in which said X-ray generator produces said beam and another beam of X-rays each directed toward one of said objects to be partially absorbed thereby and to produce a transmitted beam transmitted therethrough, said radiation-sensitive means at said given position being within one transmitted beam to be responsive to the radiation transmitted by the corresponding object, and including another radiation-sensitive means in the other transmitted beam to be responsive to the radiation transmitted by the corresponding object, and means for comparing the outputs of said. two radiation-sensitive means to obtain a measure of the disconformity between said test object and said standard object, said control means operatively connecting said voltage control device to at least one of said radiation-sensitive means.

6. Acombination as dened in claim 1 for the comparative inspection of a test object and a transmitted beam of. said standard object tol maintain the intensity of this transmitted beam substantially constant, there being another radiation-sensitive means in the transmitted beam of thetest object, and including means for comparing the outputs cf sai two radiationsensitive means to obtain a measure ci the dis-` conformity between; said test object and said standard object.

71 combination as deiined in claim c for the testing of a test object or" variable X-ray opacity throughout its length, and including for progressively vnoving said test object relative to its X-ray beam.

8. A combination as deined in claim 6 for the testing of test and sample objects each of diiferent iii-ray opacity at dinerent sections thereof, and including means for synchronously both said objects relative to their respective X-ray beams to move the sections of both objects sequentially through the respective X-ray beams, said comparing means including circuit means for producing a potential varying with the disconioimity between the test object and the standard object at corresponding sections thereof.

9, A combination as dened in claim in which saidl comparing means includes a bridge circuit connected to said two radiation-sensitive means, said' bridge circuit including means for producing a potential varying with the disccnormity between` said test object and said standard object.

1Q. A combination as dened in claim 6 in which each o said radiation-sensitive means is an ionization chamber, and in which said comparing means includes a bridge circuit connected to said ionization chambers, said bridge circuit including means for producing a potential varying with the disconformity between said test ob ject and said standard object.

11. A combination as defined in claim 6 in which the voltage control device comprises a variable impedance including a winding connected to said X-ray generator to vary the voltage applied thereto and a control winding, said 16 windings being inductively coupled, and in: whichthe control means includes means for changing the current flowing through said control winding in response to changes in radiation intensity rel ceived by said radiation-sensitive means at said given position.

12. A combination as defined i;

variable impedance including a winding connected to said X-ray generator to vary the voltapplied thereto and a control winding, said windings being inductively coup-led, and in .'ch the control means includes a grid-controlled tube for varying the current flowing through said control winding, and means operatively connect ing the grid of said tube to said radiation-sensitive means at said given position.

13. A combination as defined in claim 6 in which the voltage control device comprises a variable impedance including a winding connected to said X-ray generator to vary the voitapplied thereto and a control winding, said windings being inductively coupled, and in which the control means includes a grid-controlled tube for varying the current flowing through said con` trol winding, and a phase-shifting network operatively associated with said radiation-sensi tive means at said given position for controlling the grid voltage oi said tube.

HAROLD D. ROOF.

REFERENCES CTED rhe following references are oi record in the aile of this patent:

" UNITED sTArEs PATENTS Name Date Failla Nov. 2, 193? Trost Nov. 19, 194i) Bischoff et al. May 6, 1941 Weisglass May 18, i943 Morgan et al. viay 28, 1946 Number 2,i'i97,76-Q 2,222,451 2,249,478 2,319,378 2,401,289

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